As photosensitive materials of known electrophotographic photoconductors (also called "photoconductors"), there have been used inorganic photoconductive substances, such as selenium and selenium alloys, inorganic photoconductive substances, such as zinc oxide and cadmium sulfide, dispersed in a resin binder, organic photoconductive substances, such as poly-N-vinylcarbazole and polyvinyl anthracene, and organic photoconductive substances, such as a phthalocyanine compound and bisazo compound, dispersed in a resin binder or subjected to vacuum deposition, for example.
The electrophotographic photoconductors are required to have functions of maintaining a surface charge in the dark, generating charges upon receipt of light, and transporting the charges upon receipt of light. The known electrophotographic photoconductors include so-called single-layer type photoconductors having these functions in a single layer, and so-called function-separated laminated-layer type photoconductors each having a first layer that mainly serves to generate charges upon receipt of light, and a second layer that serves to maintain the surface charge in the dark and transport charges upon receipt of light.
The above types of electrophotographic conductors are used to form images by known electrophotographic methods, such as the Carlson method. The image formation by this method may be performed by charging the photoconductor in the dark by a corona discharge, forming a desired electrostatic latent image, such as characters or drawing of an original, on the charged surface of the photoconductor, developing the thus formed electrostatic latent image by means of toner particles, transferring and fixing the toner particles representing the desired image onto a support, such as paper. After the toner transfer, remaining toner particles are removed by cleaning, and any residual electrostatic charges are removed by erase exposures, so that the photoconductor can be used again.
In recent years, electrophotographic photoconductors using an organic substance have been put to practical use, in view of its advantageous characteristics, such as flexibility, thermal stability and film-forming capability. For example, a photoconductor formed of poly-N-vinylcarbazole and 2,4,7-trifluorene-9-one (as disclosed in U.S. Pat. No. 3,484,237), photoconductor containing an organic pigment as a major component (as disclosed in laid-open Japanese Patent Publication No. 47-37543), and a photoconductor containing as a major component an eutectic complex of a dye and a resin (as disclosed in laid-open Japanese Patent Publication No. 47-10785) have been proposed.
In the meantime, the function-separated laminated-layer type photoconductor that consists of a charge generation layer containing a charge generating substance and a charge transport layer containing a charge transport substance has been widely used in these days. In particular, numerous negative charge type photoconductors have been proposed wherein the charge generation layer is formed by vapor-depositing an organic pigment as a charge generating substance on a layer, or dispersing an organic pigment in a resin, and the charge transport layer is formed by dispersing an organic, low molecular weight compound as a charge transport substance in a resin.
Although organic substances have many advantageous properties that are not possessed by inorganic substances, such organic substances that satisfy all of characteristics electrophotographic photoconductors are required to exhibit have not been available. Namely, the photoconductor using the organic substance suffers from deterioration of the quality of images due to reduction in the potential of its charged surface after repeated use, increase in the remaining potential, and changes in the sensitivity, for example. Although not all of the causes for the deterioration have not been revealed, decomposition of the charge transport substance due repeated exposure to image light, light of erase exposure lamp, exposure to external light during maintenance, may be considered as some of the causes. It has been therefore proposed to add a dye or ultraviolet-ray absorbent to a surface protective layer or photosensitive layer so as to prevent deterioration due to these lights. For example, a dye or ultraviolet-ray absorbent having a light absorption characteristic that covers an absorbing wavelength range of the charge transport layer may be added to the surface protective layer, as disclosed in laid-open Japanese Patent Publication No. 58-160957, or a yellow dye may be added into the charge transport layer, as disclosed in laid-open Japanese Patent Publication No. 58-163946. With these known techniques, however, satisfactory effects have not been achieved, and the addition of such a dye or ultraviolet-ray absorbent may result in other problems, such as reduction in the sensitivity or an increase in the residual potential.